When dinosaurs roamed the Western Isles

The Isle of Skye off the northwest coast of Scotland is known largely as a prime tourist destination, such as Dunvegan Castle with a real clan chief (The MacLeod of MacLeod) and its Faerie Flag; Britain’s only truly challenging mountains of the Black Cuillin; and, of course, the romantic connection with the Young Pretender, Charles Edward Stuart and his escape, in drag, from the clutches of the Duke ‘Butcher’ Cumberland, hence the Skye Boat Song. Geologists know it best for its flood basalts with classic stepped topography and the exhumed guts of a massive central volcano (the Cuillin), relics of the Palaeocene-Eocene (62 to 54 Ma) North Atlantic Large Igneous Province. The spectacular Loch Coruisk, a glacial corrie drowned by the sea, exposes the deepest part of the main magma chamber. It is also the lair of Scotland’s lesser known Monster, the dread Each Uisge (Water Horse). Yet evidence is emerging for the former presence in the Hebrides of other, more tangible monsters.

Skye’s great volcanic edifice rests on Mesozoic sedimentary rocks including shallow-water muddy limestones of the Great Estuarine Group of Middle Jurassic (Bathonian, 174–164 Ma) age. For dinosaur specialists this is of the time when meat-eating theropods and herbivorous sauropods began growing to colossal sizes. Yet the Bathonian is notable for its global paucity in well exposed terrestrial and near-shore sedimentary sequences. Easily accessible, the Skye Bathonian sequence is much visited and has yielded a rich, though generally fragmentary fauna. A group of recent visiting palaeontologists from the University of Edinburgh, the Chinese Academy of Sciences and Skye’s Staffin Museum have discovered an extensive tract of wave-cut platform on the east shore of the Trotternish Peninsula where lagoonal carbonate muds were trampled by several dinosaurs that left around 50 tracks (dePolo, P.E. et al. 2018. A sauropod-dominated tracksite from Rubha nam Brathairean (Brothers’ Point), Isle of Skye, Scotland. Scottish Journal of Geology, online; doi:10.1144/sjg2017-016).

Some are of medium-sized sauropods (either Parabrontopodus or Breviparopus – both names for footprints rather than any genus of dinosaur) whose crudely elephant-like footprints are up to 0.5 m across (the largest, from Western Australia, are about 1.7 m across). Although there are fragmentary dinosaur bones from the same strata, assigning the footprint to a known species is not possible. However, foot size can be used to estimate how high the creatures’ hips stood (2 to 2.5 m): hefty beasts but not the true giants of later times A variety of three-toed, clawed, somewhat bird-like, footprints also occur. They are assigned to probably bipedal carnivores or theropods. Variation in foot size suggests a range of hip-height from about 0.9 to 2 metres, so these carnivores would have been pretty formidable.

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A friend notes: “these are clearly not simple mudstone prints. Rather they appear to have ben through some sort of fill and replacement, even perhaps a deep burial and re-exposure process. Can you elaborate a bit?”

Found answer in Guardian report: “Around 170m years ago, shortly after the supercontinent Pangaea began to break up, the land that is now Skye was part of a smaller subtropical island, far closer to the equator, and replete with beaches, rivers and lagoons.”.

In fact at 170 Ma the North Atlantic had not yet begun to open, that being associated with the ~60 Ma North Atlantic Igneous Province which initiated sea-floor spreading and the N Atlantic Ridge. What is now Scotland was close to Greenland in the Middle Jurassic, though at much lower latitudes. The top-right image at http://jan.ucc.nau.edu/~rcb7/regionaltext.html shows N Atlantic palaeogeography at that time – the British Islands’ outline is easily visible, with Scotland shown as land. Skye would have been just offshore and the sediments there suggest a lagoonal environment in which the dinosaurs romped. The reconstruction has been put together over many years by Ron Blakey of Northern Arizona University and until he retired all the reconstructions were available free of charge. Now you have to follow the links to a commercial vending site. But the full size images are real works of art and done with excruciatingly painstaking precision and attention to the literature. I’d say they are the best.

Paige comments: Your friend is exactly right in observing that these tracks are not made in a typical mudstone!

When it comes to the depositional environment, the tracks were formed in a low-energy lagoon and are generally preserved today as impressions into shaley limestone. Later, additional limestone layers were laid down at the site and in-filled the impressions. Those layers form the casts that we can still observe for some of the tracks today. In some cases at this site, the cast remains while the surrounding impression which it was originally infilling has been almost completely eroded. These rocks were deposited during the Middle Jurassic. Later, likely during the Paleogene, a sill was intruded immediately below the track bearing layer and the surrounding rocks were baked. The low-level contact metamorphism of the track-bearing layers definitely makes for some interesting looking exposures!